Presently manufactured lead frames, particularly Quad Flat Package devices, have the die mount pad supported by flat tie bars. The tie bars are attached between the corners of the die mount pad and the lead frame which may form, in part, the dam bars that hold the leads spaced at desired intervals during the package molding process. An example of this prior art structure is shown in U.S. Pat. No. 5,285,104 in which the corners of the die mount pad are attached by tie bars to the lead frame.
Another example of tie bars extending from the die attach area is shown in U.S. Pat. No. 5,429,992. In this example, a die pad is not used. The tie bars are connected to an X-type or cross shaped structure which supports the semiconductor die.
Still other examples of lead frames utilize only two tie bars to support the semiconductor die mount pad. An example of this structure is shown in U.S. Pat. No. 5,096,852, wherein the die mount pad is supported by two diagonally opposite tie bars extending out opposites sides of the die mount pad. A similar structure is shown in U.S. Pat. No. 5,548,884.
U.S. Pat. No. 5,517,056 also shows the lead frame structure where the die mount pad is attached to the lead frame by tie bars extending from the corners of the die mount pad.
The prior art, as indicated above, utilizes two to four flat tie bars. The tie bars extend from the corners of the mount die pad when four die pads are used, and when two tie bars are used, the tie bars extend from two opposite sides of the die mount pad. These flat tie bar configurations do not provide adequate support to the die mount pad during molding. Often, during the molding process of the package, the die mount pad will shift upward or downward to the surface of the package. When this happens, the device is rejected. The movement is caused by the pressure of the mold compound on the surface of the die or die mount pad, and the flexing of the tie bars. This problem is common, particularly in thin packages. In order to prevent the flexing of the tie bars, the bars need to be strengthen.